Spring Events中ApplicationEventPublisher空指针异常问题求助

我是Spring和SpringBoot初学者，正在学习Spring Events。调用publishEvent()方法时反复出现ApplicationEventPublisher对象的空指针异常。

事件类代码 #

public class EventClass extends ApplicationEvent {

    private String message;

    public EventClass(Object source, String message) {
        super(source);
        this.message = message;
    }

    public String getMessage() {
        return message;
    }
}

事件发布类代码 #

@Service
public class PublishEvents {

    @Autowired
    private ApplicationContext applicationContext;

    public void publishEvent(String message){
        System.out.println("Publishing events...");
        EventClass event = new EventClass(this,message);
        applicationContext.publishEvent(event);
    }
}

监听器类代码 #

@Component
public class ListenerClass implements ApplicationListener<EventClass> {

    @Override
    public void onApplicationEvent(EventClass event) {
        System.out.println("Listener received messages..." + event.getMessage());
    }
}

主类代码（存在问题的版本） #

@SpringBootApplication
public class SpringeventsApplication {

    public static void main(String[] args) {
        SpringApplication.run(SpringeventsApplication.class, args);
        springEvents();
    }

    public static void springEvents(){
        PublishEvents publishMessage = new PublishEvents();
        publishMessage.publishEvent("Hi I think this works...");
    }

}

问题原因 #

核心问题是手动通过new PublishEvents()创建对象，这个对象不在Spring容器的管理范围内，Spring不会为其注入@Autowired标记的ApplicationContext，因此调用publishEvent()时会触发空指针异常。

解决方案 #

1. 修正主类代码，从Spring容器获取Bean

不要手动实例化PublishEvents，而是从Spring应用上下文中获取容器管理的Bean：

@SpringBootApplication
public class SpringeventsApplication {

    public static void main(String[] args) {
        // 获取Spring应用上下文
        ConfigurableApplicationContext context = SpringApplication.run(SpringeventsApplication.class, args);
        // 从容器中获取PublishEvents实例
        PublishEvents publishMessage = context.getBean(PublishEvents.class);
        publishMessage.publishEvent("Hi I think this works...");
    }
}

2. 优化事件发布类：直接注入ApplicationEventPublisher

ApplicationContext继承了ApplicationEventPublisher，但直接注入ApplicationEventPublisher更贴合事件发布的单一职责，代码更清晰：

@Service
public class PublishEvents {

    @Autowired
    private ApplicationEventPublisher eventPublisher;

    public void publishEvent(String message){
        System.out.println("Publishing events...");
        EventClass event = new EventClass(this, message);
        eventPublisher.publishEvent(event);
    }
}

3. 简化监听器：使用@EventListener注解

无需实现ApplicationListener接口，直接用@EventListener标记处理事件的方法，代码更简洁灵活：

@Component
public class ListenerClass {

    @EventListener
    public void handleEvent(EventClass event) {
        System.out.println("Listener received messages..." + event.getMessage());
    }
}

内容的提问来源于stack exchange，提问作者Meghna Sinha